code: def test_gaussian_process_missing_values(self):
      ts = TimeSeries.from_values(np.ones(6))
  
      kernel = RBF(length_scale_bounds=(1e-3, 1e10))
      gpf = GaussianProcessFilter(kernel=kernel)
      filtered_values = gpf.filter(ts).values()
      np.testing.assert_allclose(
          filtered_values, np.ones_like(filtered_values)
      )
  
imports: import numpy
  import pandas
  from nfoursid import kalman, state_space
  from sklearn.gaussian_process.kernels import RBF, ExpSineSquared
  from darts import TimeSeries
  from darts.metrics import rmse
  from darts.models.filtering.gaussian_process_filter import GaussianProcessFilter
  from darts.models.filtering.kalman_filter import KalmanFilter
  from darts.models.filtering.moving_average_filter import MovingAverageFilter
  from darts.utils import timeseries_generation
label_desc: Use image node_id 3 for calling the TestGaussianProcessFilter obj's underlying member method code with example usage: obj.test_gaussian_process_missing_values() without return types